Four fair coins are tossed at once. What is the probability of obtaining 3 heads and one tail? 2

There are 16 possible outcomes, all equally likely. There's only one way to get four heads and only one way to get no heads. That leaves 14 ways to get two or three heads.

There's four ways to get three heads. The first coin can be the tail, the second can be the tail, the third can be the tail, or the fourth can be the tail.

There's also four ways to get one head. The first coin can be the head, the second can be the head, the third can be the head, or the fourth can be the head.

That leaves six ways left to get exactly two heads. So 6/16 or 3/8.